As be seen by reference to the following U.S. Pat. Nos. 4,072,353; 3,565,488; 4,311,343; 4,099,784; and 4,152,028 the prior art is replete with myriad and diverse hydraulically actuated rock splitting arrangements.
Mechanical rock excavation tools like this invention fall under the classification of rock splitters. Rock splitting tools normally apply loads to the rock from within a predrilled hole. The majority of these tools produce just radial loads and in operation they split rock along some roughly planar surface that contains the drill-hole axis.
Some rock splitters, including this invention, produce both radial and axial loads. Under the influence of this type of tool, the rock is split perpendicular to the drill-hole axis rather than along it. In use, this additional loading action makes it possible to excavate a plug of material from a mass.
In the design of this type of splitter, three coaxial mechanical rock breaking elements that are affixed to a special hydraulic cylinder that contains two independent double-acting pistons are placed within a predrilled hole. Of the three coaxial elements, two called the wedge and feather combine to produce the radial load. The wedge element is affixed to one of the hydraulic cylinders' two pistons and the feather is affixed to the cylinder body itself. The action of the wedge causes it to be withdrawn into the feather element.
This action produces two problems. The first is a result of the unique design of this hydraulic cylinder. The two pistons it contains operate independently traveling toward one another within the cylinder. Because sufficient room for movement is provided for each piston, it is possible for the piston, to which the wedge is affixed, to travel too far. This over travel causes the end of the wedge to be withdrawn past the end of the feather. This action produces brittle, or ductile, failure of the feather.
The second problem is in the design of the wedge and feather elements contacting surfaces. The wedge element is designed with an end that has an increasing conical profile. Correspondingly, the feather has an end that has a matching conical profile on its interior surface where the two elements contact before the wedge is withdrawn. The feather, being a split piece, is free to expand radially, contacting and loading the drill-hole wall.
This radial loading action is accomplished by withdrawing the conical-shaped end of the wedge into the feather's segments. A problem arises because the contacting conical surfaces of these two elements become mismatched as the wedge is being withdrawn. The relative movement of the wedge to the feather causes each feather segment to contact an ever increasing conic diameter on the wedge. This mismatching results in the formation of a gap between these pieces. Because the feather's segments are in effect being squeezed between the drill-hole wall and the wedge, the gap allows bending to occur in the segmented sections of the feather. This bending produces great stress in these segments resulting in their failure.
The feather has an additional design problem stemming from the exterior cylindrical geometry of its end. This design promotes a uniform stress distribution across its contact area with the drill-hole wall. This nonstress focusing design allows the disc-shaped fracture that develops in the rock to initiate anywhere along the feather's cylindrical exterior profile, limiting the operator's control over the initiation point. This cylindrical feather design also makes poor use of the load it transfers. The uniform distribution it produces creates rock stresses that are minimized rather than optimized resulting in a tool that is less effective than it could be.